In the manufacture of semifinished products, a strand or slabs is/are generally cast from liquid rolling stock and subsequently processed further to form the semifinished product. For this purpose, they are generally processed by a hot and/or cold rolling mill.
When rolling stock is introduced or threaded into a roll stand, there are often signs of wear on the working rolls of the roll stand and/or losses in throughput as result of the threading operation. The effects of wear on the working rolls are brought about by the rolling stock that is running into the roll stand hitting the lateral surface of a working roll and causing so-called “roll marks”. Depending on the dimensions of such damage to the working rolls, an immediate roll change may be required and this constitutes a high degree of roll wear.
Nowadays various methods are used in rolling mills for threading a rolling stock into a roll stand or into a rolling train in order to avoid such damage to the working rolls.
For example, it is known from the operator side to thread a rolling stock into a rolling train in such a way that at first all the roll stands of the rolling train are open, a rolling stock is introduced into the entire rolling train, subsequently all the roll stands are closed, and then a rolling operation, preferably a continuous rolling operation, is commenced. This has the consequence that a large amount of rejected rolling stock is created with each threading operation. One instance of rejected rolling stock in such a case is the length of the entire rolling train for each threading operation. In particular in the case of discontinuous rolling, i.e. in the case of so-called batch mills, the reduction in efficiency of the rolling train as result of such a threading operation is considerable, since there are rolling stock losses for each slab. Furthermore, this method is time-consuming, since it is carried out manually.
Alternatively, it is likewise known from the operator side to thread a rolling stock into a roll stand with the roll stand already set to a roll gap that provides the reduced thickness of the rolling stock on the outlet side. Here, the rolling stock is introduced into the roll stand very slowly, in order to keep the damage to the working rolls as low as possible. Furthermore, the circumferential speed of the working rolls is much greater than the speed of the rolling stock running in. This is so because the circumferential speed of the working rolls has already been adapted to the outlet thickness of the rolling stock from the roll stand. On the one hand, the damage to the working rolls cannot be entirely avoided here and, on the other hand, the required low speed of the rolling stock on the inlet side leads to losses in throughput.
Also known as a further measure for reducing working roll damage during introduction into a roll stand are special operating modes such as nosing and the application of lubricant, especially oil or an oil emulsion, to the rolling stock. These special operating modes likewise have an adverse effect on a throughput-optimized operating sequence and therefore similarly leads to losses in throughput.